Enya 240 V-twin. As far as size, weight and power is concerned, this is about the only one that will fit in the cowl, balance properly, and fly with authority. Check out Sky Pilot's RPM data of this engine in the Engine -> Tachometer forum.

Saito/OS 300 twins, I think, will not fit in the cowl

Saito 170 3-cylinder does not have enough power for this bird

Laser 200 & 240 V's I think does not come close in power to the Enya 240